Osmosis is a natural phenomenon in which water passes through a semipermeable barrier from the side with lower solute concentration to the higher solute concentration side. Water flow continues until chemical potential equilibrium of the solvent is established. At equilibrium, the pressure difference between the two sides of the membrane is equal to the osmotic pressure of the solution. To reverse the flow of water (solvent), a pressure difference greater than the osmotic pressure difference is applied; as a result, separation of water from the solution occurs as pure water flows from the high concentration side to the low concentration side. This phenomenon is termed reverse osmosis (it has also been referred to as hyperfiltration).
A reverse osmosis (R-O) membrane acts as the semipermeable barrier to flow in the R-O process, allowing selective passage of a particular species (solvent, usually water) while partially or completely retaining other species (solutes).